In the field of motor vehicles which are operated by means of internal combustion engines it is a general requirement that the concentration of certain regulated substances, such as unburnt residues of hydrocarbons (HC), oxides of nitrogen (NOx) and carbon monoxide (CO), in the engine's exhaust gas should be as low as possible. In today's motor vehicles equipped with gasoline engines, a purification of the exhaust gas is normally carried out by means of a catalytic converter, or catalyst, arranged in the exhaust system. In the contemporary so-called three-way catalyst, the major part of the above-mentioned substances is eliminated by means of various well-known catalytic reactions.
The catalysts commonly used today provide a very high degree of exhaust gas purification at the appropriate operating temperature of the catalyst. However, from cold start a certain time period of heating is required in order for the catalyst to reach the operating temperature at which an optimum degree of purification may be obtained. The so-called “light-off temperature” of the catalyst is approximately 200-350° C. and may be defined as the temperature at which the catalyst provides a 50% degree of purification of a certain regulated component in the exhaust gases. During the initial warm-up phase of the catalyst, which e.g. may be approximately 30-90 seconds, the catalyst cannot operate in an optimum manner as regards the elimination of the regulated substances in the exhaust gases.
In order to shorten the time required for the catalyst to reach the so-called “light-off temperature” is known to inject secondary air into the exhaust gas. This secondary air is mixed with the exhaust gas immediately downstream of the engine's exhaust valves, resulting in an oxidation of the mixture consisting of the exhaust gases and the secondary air. The oxidation process is mainly due to the hydrogen which is present in the exhaust gases. The oxidation reaction generates a high amount of heat energy which is guided through the exhaust pipe and fed to the catalyst, which subsequently becomes heated more rapidly.
A technical problem encountered with the secondary air injection systems described in the foregoing is that although the above-mentioned arrangement results in a reduced time taken until the so-called “light-off temperature” of the catalyst is reached, it suffers from the drawback that the performance of the engine is easily negatively affected. This is among other things due to the fact that so-called “breathing” between the cylinders of the internal combustion engine tends to occur, resulting in low end torque loss.
A previously known way of addressing this problem has been to separate the exhaust banks of cylinders likely to cause this problem and to inject secondary air into the exhaust gas of the respective exhaust banks separately. However, in order to reduce the cost and complexity of such a secondary air injection system usually a common secondary air injection pump is used, the air of which is then selectively fed into the respective exhaust banks using a respective complex and rather expensive shut-off valve. For packaging reasons the shut-off valves are usually arranged in the direct vicinity of the exhaust banks, for which reason the material used for the shut-off valves must be thermally safe, i.e. able to withstand the high temperatures associated with the exhaust banks. This tends to further increase the cost of the shut-off valves.